1. Field of the Invention
This invention relates to extruders of the type which employ single screws to mix material to be extruded. Specifically this invention relates to the use of specific patterns of undercut barriers to increase interfacial area between material elements and heat/mass transfers to achieve greater distributive and dispersive mixing.
2. Description of the Related Art
Single screw extruders are widely used in the plastics industry as mixers and pumps. The simplest design consists of a screw, which rotates inside a close fitting cylindrical barrel. The screw typically includes a feed section, a transition section, and a metering section. Virtually all studies of single screw extruders are based on the unwound representation of the screw channel. Referring to FIG. 1 the relative motion of the screw and the barrel appears as a plate which moves diagonally on top of the channel in the direction of the arrow marked "v". The fluid flow in the channel can be decomposed into two components, namely, a cross flow in the x.sub.1 -x .sub.2 plane and an axial flow in the x .sub.3 direction. The axial flow pumps the materials forward through the screw and the cross flow mixes the material. However, depending on the characteristics of the material the mixing in such a prior art screw is poor.
With respect to FIG. 2, typically, a mixing section such as a Maddock mixing section, a pin mixing section, a pineapple mixing section, a blister ring, and so on, is added to the screw to increase its dispersive and/or distributive mixing capability. However, these mixing sections are typically relatively short compared to the length of the screw. The screw section responsible for mixing is typically characterized by its closeness to the outer barrel to generate the high shear stress required for a dispersive mixing (e.g., Maddock section and blister ring), or by a large number of small units affixed on the root of the screw for a distributive mixing (e.g. pin and pineapple mixing sections).
Another prior art mixing screw, the barrier screw, is designed based on the solid melt distribution as it is conveyed down the metering section. The barrier screw has two channels, used to separate melt from solid, of varying width separated by an undercut barrier flight. The width of the channel is proportional to the amount of solid or melt. Initially, as pellets start to melt, the melt pool is pushed into the narrow melt channel. The melt channel grows larger as more melt is collected. The Barr ET screws (namely, a Barr ET Barrier screw (manufactured by Robert Barr Incorporated of Virginia Beach, Va.) is claimed to have 30 to 50% better melting rate through mixing of pellets with fresh melt.
A prior art modified double wave screw typically has two equal width channels separated by an undercut barrier flight wherein the roots of each channel go up and down like a wave. This continually reverses and forces melted plastic back and forth across the barrier. The material in the channel is alternately subjected to high shear and then low shear as it crosses between the barrier and the barrel wall. Usually these double wave mixing sections are located in the metering section where the plastic has already been melted and consist of 3 to 4 waves. Although double wave screws increase the mixing of material as it flows through an extruder, with certain types of materials the level of mixing is still less than optimal.
Many of the extruder screws of the prior art rely on molecular diffusion to affect mixing, while smart extruder screws or other type of blenders of the prior art have been designed to augment mixing and exploit the capabilities of such devices to generate new interfacial area between the polymeric components. The degree of mixing, therefore, is a strong function of rate of creation of polymer-polymer interface, which in turn depends on the design of the extruder or blender and viscosity and interfacial tension between the components. The mechanism by which new interfaces are repeatedly created is known as "distributive" mixing. In distributive mixing, the length scale of each component is dramatically reduced to thin striations due to repeated stretching and folding of each phase with the resultant mixture being a collection of alternating thin striations of the phases. Note that a distributively mixed material still has two-phases, though on a very microscopic scale. On a macroscopic scale, however, the material looks "mixed" and the various properties are now governed by the contributions from both the components.
In the case of blending of immiscible polymeric components, the objective is to produce dispersions of fine droplets/particles of the minor component in the continuous phase formed by the major component. The mixing mechanism that produces this dispersed-continuous morphology is popularly known as "dispersive mixing". In this case, the interfacial tension between the components is finite.
Dispersive mixing occurs in two stages. First, the minor component stretches into thin striations due to a "distributive" mixing mechanism of the blending device. The stretching continues until a critical local length scale is reached, such that the interfacial forces become more important than the viscous forces and the striations break into fine droplets. These droplets may retain shape and size, or, in turn, may deform, stretch into striations, and break further into finer droplets. The stretching and breakup processes repeat until an asymptotic droplet size distribution is reached such that no further breakup is possible. Some secondary processes, such as coalescence of two small drops into a larger drop compete with the breakup process and influence the final size distribution. Secondly, these droplets solidify due to freezing of the melt and become dispersed particles. The physical properties of component materials, operating conditions of blending, and design of blender, all have direct bearing on the quality of mixing of immiscible blends.
A typical two-stage single extruder screw serves three functions: (1) feeding; (2) melting and mixing; and (3) metering. FIG. 3 is a typical two stage single extruder screw of the prior art including a number of zones with zones labeled as feeding, transition, first metering, venting, and second metering. Melting of solids which starts in the transition zone, also called compression zone, should end early in the metering zone. Some degree of mixing between unmelted solids with the melt may occur in the transition zone itself. Many commercially available extruder screws of the prior art are specially designed to perform this specific function and guarantee very early melting, an example being the Barr ET screw as described herein above. Another popular screw, the barrier screw, separates melts from unmelted solid so that the solid is always subjected to high shear forces caused by friction with the barrel wall and therefore melts rapidly. The first metering section simply conveys the melt by drag flow into the venting zone and the second metering zone pumps the melt into a stranding die.
There are no inherent mechanisms built into the screw design of the prior art to ensure mixing of two or more polymeric materials. FIG. 4 shows cross-channel flow in the metering section of an unwound screw of the prior art wherein the motion is mostly circulatory. There is no reorientation mechanism and therefore, two elements of fluid A and B will most likely never mix with each other, except through molecular diffusion. In some cases it is advantageous to provide auxiliary mixing sections at the end of first metering zone to improve mixing. Examples of such auxiliary mixing sections are shown in FIG. 2 and include a blister ring, an Egan mixing unit, a pineapple mixing section, a cavity transfer mixer, a Dulmage mixing section, among others. Distributive and dispersive mixing are produced with the aid of these auxiliary mixing sections as the melt is forced through the small gap between the barrel and the mixing section. The typical length of these auxiliary mixing sections range from 2-4 times the diameter of the screw. The degree of mixing achievable from these sections is usually small as the melt spends only a short time in the mixing zone. In addition, the auxiliary mixing sections typically cause a large pressure drop.
Another screw design of the prior art aimed at improving distributive mixing in a single extruder screw includes a series of undercut baffles placed along the screw channels to disrupt streamlines of the cross-channel flow. The baffle locations are alternated periodically to produced chaotic mixing of the melt. There are several limitations in such an extruder screw. First, if the channels are not completely filled, only a part of the channel cross-section is available for chaotic mixing; therefore reducing the extent of distributive mixing. This is especially true for polymers available in the form of low bulk density powder. Secondly, the aspect ratio of the screw channel must lie between 4 and 6 for realization of appreciable chaotic mixing. This poses serious restriction on the available flow area and throughput rates as the undercut baffles also occupy some physical space. Thirdly, no dispersive mixing mechanisms are provided by the undercut baffles of such a design. As stated herein above it is advantageous for an extruder screw to be able to produce both dispersive and distributive mixing.
It is apparent from the above there exists a need in the art for an extruder which is capable of increased dispersive and distributive mixing. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.